A prior art semiconductor device that changes the position of and modulates a signal light in response to an electrical signal is shown in a perspective, schematic view in FIG. 5. The device of FIG. 5 includes two opposed, generally planar, spaced apart light guide layers 1 and 2. The light guide layers are gallium arsenide (GaAs) or aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As) where x is about 0.1. The guide layers 1 and 2 sandwich and contact a spacer layer 3 of p-type Al.sub.x Ga.sub.1-x As. The three layer structure is, in turn, laminated between first and second n-type Al.sub.x Ga.sub.1-x As cladding layers 10 and 20 where x is about 0.3. The five laminated layers are disposed on a gallium arsenide substrate 101. Voltage sources 11 and 21 are provided to apply variable reverse bias voltages across spacer layer 3 and second cladding layer 20 and across spacer layer 3 and first cladding layer 10, respectively. The refractive indices of the light guide layers 1 and 2 are higher than the refractive index of the spacer layer 3 and of the first and second cladding layers 10 and 20 to confine light to the light guide layers 1 and 2. Usually, the device is compositionally symmetrical, i.e., the compositions of the light guide layers 1 and 2 are the same and the compositions of the cladding layers 10 and 20 are the same.
The device of FIG. 5 is prepared by conventional techniques. The n-type Al.sub.x Ga.sub.1-x As first cladding layer 10 is grown to a thickness of about 2 microns on the GaAs substrate 101 by a conventional technique, such as liquid phase epitaxy (LPE), metal organic chemical vapor deposition (MOVCD), or molecular beam epitaxy (MBE). The other layers are grown by similar techniques. The GaAs or Al.sub.x Ga.sub.1-x As first light guide layer 1 is grown to a thickness of about 0.2 micron, the p-type Al.sub.x Ga.sub.1-x As spacer layer is grown to a thickness of about 0.2 to 1.0 micron, and the guide layer 2 and second cladding layer 20 are grown to be symmetrical in thickness and composition to the first guide layer 1 and the first cladding layer 10, respectively.
After the layers are grown, electrical contacts are made to the substrate 101, the second cladding layer 20, and the spacer layer 3 to apply bias voltages from the voltage sources 11 and 21. Electrodes can be applied to the substrate and the second cladding layer using conventional techniques. However, in order to make an electrical contact to the spacer layer, an opening must be etched through second cladding layer 20 and guide layer 2 to expose the spacer layer 3. Controlling the depth of etching to ensure exposure of the spacer layer 3 having a thickness of only 0.2 to 1.0 micron without penetrating entirely through that layer presents difficult processing problems. Additional problems occur in metallizing only the spacer layer 3 as part of the forming a contact while avoiding short-circuiting of the spacer layer 3 to the light guide layer 2.
In operation, the device of FIG. 5 selectively switches a signal light beam from guide layer 1 to guide layer 2, or vice versa, in response to the strength of the electrical bias signals provided by voltage sources 11 and 21. An input signal light beam incident on the light guide layer 1 is indicated by arrow A in FIG. 5. The output signal light beam from the opposite end of the device is indicated by arrow B in FIG. 5. As schematically illustrated in that figure, the output signal light beam B can emerge from guide layer 2. It is known in the art that light guide layers 1 and 2 are optically coupled by the electromagnetic field associated with the signal light beam. The amount of the optical coupling between guide layers 1 and 2 depends upon a number of variables including the wavelength of the signal light beam, the material in which the signal light beam is propagating, and the length of the path along which the signal light beam travels. When that path is a predetermined length, called the critical length, the signal light beam entering guide layer 1 is totally transferred to guide layer 2. If the length of the device is chosen to be coincident with the critical length, then a signal light beam input into guide layer 1 will emerge from guide layer 2 at the opposite end of the device. A typical device of the type shown in FIG. 5 has a length between the light incident and output surfaces of 0.5 to 1.0 millimeter, the width of the light incident face is typically 2 microns, and the height of the device is typically 5 microns.
When reverse bias voltages are applied across the light guide layers 1 and 2 with respect to spacer layer 3, the refractive indices of the spacer layer 3 and of the light guide layers 1 and 2 are changed. The reverse bias voltages alter the width of the depletion regions created by the pn junctions and produce electro-optic effects, such as the Franz-Keldish effect, that change the refractive indices, thereby altering the critical length of the structure. Thus, by switching the reverse bias voltage applied to the device, the presence or absence of a signal light beam output from light guide layer 2 and derived from a signal light beam applied to light guide layer 1 can be controlled. By appropriately modulating the bias voltages, the signal light beam output emerging from light guide layer 1 or light guide layer 2 derived from a signal light beam applied to light guide layer 1 can be modulated.
In addition to the difficulties of fabricating the device of FIG. 5, that device is incompatible with other semiconductor devices that are controlled by light. Since the device of FIG. 5 requires the application of a reverse bias voltage to function, electrical leads and connections that are not otherwise necessary, for example, in a light-controlled computer, are required to operate the device of FIG. 5. Voltages as high as about 10 volts are required for reverse biasing the device, a voltage level that is undesirably high in many applications, particularly in a light-driven computer.